A field-effect transistor is one of the most widely-used semiconductor elements. A variety of materials are used for field-effect transistors according to their uses. In particular, semiconductor materials including silicon are frequently used.
The field-effect transistor using silicon has characteristics satisfying the needs with respect to a variety of uses. For example, single crystal silicon is used for an integrated circuit or the like which needs to be operated at a high speed, whereby the need with respect to the integrated circuit is satisfied. Further, amorphous silicon is used for an object which needs a large area, such as a display device, whereby the need with respect to the object can be satisfied.
As described above, silicon is highly versatile and can be used for various purposes. However, in recent years, semiconductor materials tend to be expected to have higher performance as well as versatility. For example, in terms of improving performance of a large-area display device, in order to realize high-speed operation of a switching element, a semiconductor material which facilitates increase of the area of a display device and has higher performance than amorphous silicon is needed.
Under such conditions, a technique relating to a field-effect transistor (also called an FET) using an oxide semiconductor has attracted attention. For example, in Patent Document 1, a transparent thin film field-effect transistor using a homologous compound InMO3(ZnO)m (M is In, Fe, Ga, or Al, and m is an integer number of greater than or equal to 1 and less than 50) is disclosed.
In addition, in Patent Document 2, a field-effect transistor is disclosed in which an amorphous oxide semiconductor whose electron carrier concentration is less than 1018/cm3 and which contains In, Ga, and Zn is used. Note that in Patent Document 2, the ratio of In:Ga:Zn equals to 1:1:m (m<6) in the amorphous oxide semiconductor.
Further, in Patent Document 3, a field-effect transistor is disclosed in which an amorphous oxide semiconductor including a microcrystal is used for an active layer.
[Reference]
[Patent Document]
    [Patent Document 1] Japanese Published Patent Application No. 2004-103957    [Patent Document 2] PCT International Publication No. 05/088726    [Patent Document 3] Japanese Published Patent Application No. 2006-165529